There's no real limit on the number of users from a licensing standpoint. The real limitations have more to do with database/application design and available environment resources, which are limited in Express.

Express can only use the lesser of 1 CPU or 4 cores (in other words in a system with two 2-core CPUs it will only use 1 of the two CPUs) and has a max limit of 1.4GB usable RAM.